Currently employed turbine rotor alloys are designed for either high-temperature or low-temperature applications. High-temperature alloys, such as Cr-Mo-V, have superior creep and fatigue properties, while low-temperature alloys, such as Ni-Cr-Mo-V, have better yield strength. Unfortunately, turbine rotors are often exposed to both high and low temperatures at the same time, and require the superior properties of both high and low temperature alloys.
One attempt by the industry for overcoming the weaknesses in selecting a single alloy system, is to employ half sections of high-temperature Cr-Mo-V and low-temperature Ni-Cr-Mo-V alloys joined together by welding. See Clark et al., U.S. Pat. No. 4,633,554, which is hereby incorporated by reference. This technique, however, is more time consuming and generally more expensive than using a single forging. Welding defects can also occur during the manufacturing of such a rotor, which could later cause failure during service.
While attempting to develop alloys having improved high-temperature strength, toughness and corrosion resistance for pressure vessels, the Oak Ridge National Laboratory has identified a modified 9Cr-1Mo alloy (hereinafter "9Cr-1Mo mod."). See Patriarca, "Modified 9Cr-1Mo Steel Technical Program and Data Package for Use in ASME Section I and VIII Design Analysis", pp. 1, 21-48 ORNL Technology Transfer Meeting, 7 Apr. 1982, which is hereby incorporated by reference. This new alloy, now commercially available as ASTM A182, F91 has a room temperature yield strength of about 60 ksi (415 MPa) with a 20% elongation and a 40% reduction of area when normalized at about 1900.degree.-2000.degree. F. (1038.degree.-1093.degree. C.) and tempered at the specified 1350.degree. F. (732.degree. C.) minimum required temperature.
Prior studies have shown that turbine components made from the alloys of this invention exhibit preferred yield strengths for use in high temperature and pressure rotor applications. It has also been noted that by lowering the minimum A182, F91 tempering temperature of 1350.degree. F. (732.degree. C.) to a preferred range of 1275.degree.-1300.degree. F. (690.degree.-704.degree. C.), the yield strength is increased from a nominal 60 ksi (415 MPa) to a preferred 85-100 ksi (551-758 MPa). See Clark, U.S. Pat. No. 4,762,577, which is hereby incorporated by reference.
Nevertheless, there is a current need for a turbine rotor suitable for simultaneous use in high pressure and temperature and low pressure and temperature service. Such an alloy must exhibit adequate fracture toughness at about room temperature for the low temperature or ambient end of the rotor and excellent creep properties at temperatures exceeding about 800.degree. F. (427.degree. C.) at the high temperature end of the rotor.